1. Technical Field
This invention relates to polishing pads used for creating a smooth, ultra-flat surface on such items as glass, semiconductors, dielectric/metal composites and integrated circuits. It particularly relates to the bulk structure of such pads and their ability to allow optical in-situ end point detection during the polishing or planarization process.
2. Background Art
It is desirable to effect planarization of integrated circuit structures in the form of semiconductor wafers during the manufacture of multilayer integrated circuits. The planarization must be very precise, providing a wafer surface that varies from a given plane by as little as a fraction of a micron. This is usually accomplished by CMP, chemical-mechanical polishing, on an apparatus most often comprised of a rotating table, usually circular, onto which is affixed a polishing pad, a wafer carrier which presses the wafer flatly onto the polishing pad, and a means of supplying chemicals and abrasives to the polishing pad in the form of a slurry. Apparatus for polishing thin, flat semiconductor wafers are well known in the art. Such planarization apparatus are manufactured by IPEC Planar, Strausbaugh Manufacturing and the SpeedFam Corporation among others.
A particular problem encountered when planarizing semiconductor wafers on such apparatus is the determination that a wafer has been polished to the desired degree of flatness. Most end-point detection methods shown in the art rely on the change in the surface structure of the wafer as an overlying layer is removed. Thus flatness is not measured, but is only considered secondary to removal of the overlying layer. In U.S. Pat. No. 5,036,015 it is the change in friction between the wafer and the polishing pad which indicates an end-point. In U.S. Pat. No. 5,240,552 the thickness of the wafer is measured by the analysis of reflected acoustic waves. In U.S. Pat. No. 5,337,015 special electrodes underneath the polishing pad along with an electrically grounded polishing table and the use of a conductive slurry allows the dielectric layer thickness to be measured. These devices for in-situ measurement of thickness are very complicated and rely on specialized electronic circuitry to accomplish the task. Most often, instead of using a complicated in-situ method, wafers are removed from the polishing apparatus and flatness is measured using a spectroscopic device to measure the oxide film thickness. Usually, the wafer is taken out of the polishing operation before the expected end point is reached so that excess polishing does not occur. Then the wafer is reinserted into the polishing machine for polishing to the desired endpoint.
U.S. Pat. No. 5,081,796 shows a method and an apparatus for carrying the wafer while on the polishing machine out over the edge of the polishing pad so that a rapid method of measuring the oxide layer, such as laser interferometry, can be used on the underside of the wafer. This method has the disadvantage of removing pan of the wafer from the polishing process at any given time so that the wafer does not receive uniform polishing at all times. This is also true for the optical end point detection method in semiconductor planarizing polishing processes shown in U.S. Pat. No. 5,413,941. It would be very desirable to have a machine upon which such laser light measurements could be employed while the wafer is continuously under total polishing conditions.